1. Field of the Invention
The present invention relates to an optical information recording medium and particularly to a heat mode optical information recording medium.
2. Description of the Related Art
An optical information recording medium (such as DVD) has been required to have a further increased recording density for use in recording and playing-back a large amount of character information, image information and audio information. Particularly for use in picture recording of digital high-vision TV signals, studies are being carried out to provide an optical information recording medium having a higher recording density.
Under such circumstances, a blue-violet laser has been developed, and development of an optical disk system using the blue-violet laser and a high-NA pickup is now being studied. In “ISOM 2000”, Sony Corporation presented the DVR-Blue system, which uses a phase change recording medium using the blue-violet laser. The phase change recording medium of the DVR-Blue system has a layered structure which is characterized by having a 0.1 mm-thick transparent layer, called a cover layer, on a laser beam incident side thereof.
Some methods have been proposed for the formation of the 0.1 mm-thick transparent layer, including, for example, a process of bonding a transparent sheet via an adhesive layer such as an ultraviolet curable resin layer and a pressure-sensitive adhesive layer (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2000-285520, 2002-8265 and 2000-67468).
The optical information recording medium is used for TV picture recording, data storage in personal computers and the like. In many cases, the medium is used at room temperature.
In some cases, however, the interior of the drive actually has a high temperature due to the heat generated by the device itself. Particularly during continuous operation for a long time, the interior of the drive may have a temperature of 40° C. or higher. The optical disk is inevitably influenced by such a change in temperature during recording and playing-back. Therefore, reduction of a change in characteristics due to temperature change is required.
Phase change materials and organic dyes generally used for recording materials are stable and undergo little change in physical properties at a temperature from room temperature to about 100° C. In contrast, many of the ultraviolet curable resins and the pressure-sensitive adhesives for use in bonding the cover layer have a glass transition point (Tg) in a range from room temperature to about 100° C., and the adhesive layer can often undergo a change in a physical property (hardness) as the temperature increases. If the physical property changes within the temperature range of an environment in which the recording medium is used, the recording characteristics can also change within the temperature range so that stable recording and playing-back characteristics cannot be attained. In the case of some recordable (non-rewritable) optical information recording media (such as DVD-R) using an organic dye as the recording material, high modulation factor is achieved by using a difference (index of modulation) between the reflectances of an unrecorded portion and a recorded portion, which is produced after the adhesive layer of the recorded portion is deformed. Particularly in such media, if the hardness varies, susceptibility to the deformation can vary, and therefore sensitivity can vary.